Let's start with Quidditch. A small, golden, flying ball, called the Snitch, is let loose. This is the OneRing. The Seekers (one on each team) are trying to claim the Snitch for themselves. Whoever gets it, wins. Frodo is the Seeker for the good guys, and Sauron is the Seeker for the bad guys. But we have to stretch this analogy because Frodo already has the Ring but hasn't won. He just has to keep Sauron from getting it by taking it to a volcano in Sauron's backyard and destroying it. Sauron just needs to get the Ring to win. But it's pretty close. So the Seekers are the only ones that matter, really. Getting the Snitch/Ring is worth a gazillion points. They are the endgame. But. This does not three movies make, or eight in the case of Harry Potter. So.
We have everyone else. In Quidditch, there's a few Chasers, a few Beaters, and one Keeper on each team. They stay occupied by beating each other up with clubs and balls until the Seekers win the game. There are points involved, but they don't really matter because they can't surpass the endgame of Ring-getting. I see Aragorn as a Keeper for good, and Saruman as Keeper for bad. They're relatively more important than the other peons, but only because the writer said so. The only thing that's important for 'Everyone Else' to do is stay alive so that there's some people left to hang out when one side finally wins.
I think that LotR and Quidditch would work a lot more sensibly if each side just went after the Snitch/Ring as a whole group. Yes, it would be constant chaos, and the Ring is supposedly the most corrupting thing ever, but I'm reasonably sure it would condense these stories down to one movie, max.